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https://issues.apache.org/jira/browse/KAFKA-13636?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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David Jacot reassigned KAFKA-13636:
-----------------------------------
Assignee: David Jacot
> Committed offsets could be deleted during a rebalance if a group did not
> commit for a while
> -------------------------------------------------------------------------------------------
>
> Key: KAFKA-13636
> URL: https://issues.apache.org/jira/browse/KAFKA-13636
> Project: Kafka
> Issue Type: Bug
> Components: core, offset manager
> Affects Versions: 2.4.0, 2.5.1, 2.6.2, 2.7.2, 2.8.1, 3.0.0
> Reporter: Damien Gasparina
> Assignee: David Jacot
> Priority: Major
>
> The group coordinator might delete invalid offsets during a group rebalance.
> During a rebalance, the coordinator is relying on the last commit timestamp
> ({_}offsetAndMetadata.commitTimestamp{_}) instead of the last state
> modification {_}timestampt (currentStateTimestamp{_}) to detect expired
> offsets.
>
> This is relatively easy to reproduce by playing with
> group.initial.rebalance.delay.ms, offset.retention.minutes and
> offset.check.retention.interval, I uploaded an example on:
> [https://github.com/Dabz/kafka-example/tree/master/docker/offsets-retention] .
> This script does:
> * Start a broker with: offset.retention.minute=2,
> o[ffset.check.retention.interval.ms=|http://offset.check.retention.interval.ms/]1000,
> group.initial.rebalance.delay=20000
> * Produced 10 messages
> * Create a consumer group to consume 10 messages, and disable auto.commit to
> only commit a few times
> * Wait 3 minutes, then the Consumer get a {{kill -9}}
> * Restart the consumer after a few seconds
> * The consumer restart from {{auto.offset.reset}} , the offset got removed
>
> The cause is due to the GroupMetadata.scala:
> * When the group get emptied, the {{subscribedTopics}} is set to
> {{Set.empty}}
> ([https://github.com/apache/kafka/blob/trunk/core/src/main/scala/kafka/coordinator/group/GroupMetadata.scala#L520-L521])
> * When the new member joins, we add the new member right away in the group ;
> BUT the {{subscribedTopics}} is only updated once the migration is over (in
> the initNewGeneration) (which could take a while due to the
> {{{}group.initial.rebalance.delay{}}})
> * When the log cleaner got executed, {{subscribedTopics.isDefined}} returns
> true as {{Set.empty != None}} (the underlying condition)
> * Thus we enter
> [https://github.com/apache/kafka/blob/trunk/core/src/main/scala/kafka/coordinator/group/GroupMetadata.scala#L782-L785]
> with an empty {{subscribedTopics}} list and we are relying on the
> {{commitTimestamp}} regardless of the {{currentStateTimestamp}}
>
> This seem to be a regression generated by KIP-496
> https://cwiki.apache.org/confluence/display/KAFKA/KIP-496%3A+Administrative+API+to+delete+consumer+offsets#KIP496:AdministrativeAPItodeleteconsumeroffsets-ProposedChanges
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